In the processing of integrated circuits, electric contact must be made to isolated active device regions formed within a wafer substrate typically comprising monocrystalline silicon. The active device regions are connected by high electrically conductive paths or lines which are fabricated above an insulator material, which covers the substrate surface. To provide electrical connection between the conductive path a and active-device regions, an opening in the insulator is provided to enable the conductive films to contact the desired regions. Such openings are typically referred to as contact openings, or simply "contacts".
As transistor active area dimensions approached one micron in diameter, conventional process parameters resulted in intolerable increased resistance between the active region or area and the conductive layer. The principal way of reducing such contact resistance is by formation of a metal silicide atop the active area prior to application of the conductive film for formation of the conductor runner. One common metal silicide material formed is TiSi.sub.x, where x is predominantly "2". The TiSi.sub.x material is typically provided by first applying a thin layer of titanium atop the wafer which contacts the active areas within the contact openings. Thereafter, the wafer is subjected to a high temperature anneal. This causes the titanium to react with the silicon of the active area, thus forming the TiSi.sub.x. Such a process is said to be self-aligning, as the TiSi.sub.x is only formed where the titanium metal contacts the silicon active regions. The applied titanium film everywhere else overlies an insulative, and substantially non-reactive, SiO.sub.2 layer.
Ultimately, an electrically conductive contact filling material such as tungsten would be provided for making electrical connection to the contact. However, tungsten adheres poorly to TiSi.sub.x. Additionally, it is desirable to prevent intermixing of the contact filling material with a the silicide and underlying silicon. Accordingly, an intervening layer is typically provided to prevent the diffusion of the silicon and silicide with the plug filling metal, and to effectively adhere the plug filling metal to the underlying substrate. Such material is, accordingly, also electrically conductive and commonly referred to as a "barrier layer" due to the anti-diffusion properties.
One material of choice for use as a glue/diffusion barrier layer is titanium nitride. TiN is an attractive material as a contact diffusion barrier in silicon integrated circuits because it behaves as an impermeable barrier to silicon, and because the activation energy for the diffusion of other impurities is very high. TiN is also chemically thermodynamically very stable, and it exhibits typical low electrical resistivities of the transition metal carbides, borides, or nitrides.
TiN can be provided or formed on the substrate in one of the following manners: a) by evaporating Ti in an N.sub.2 ambient; b) reactively sputtering Ti in an Ar and N.sub.2 mixture; c) sputtering from a TiN target in an inert (Ar) ambient; d) sputter depositing Ti the titanium metal contacts the silicon active regions. The applied titanium film everywhere else overlies an insulative, and substantially non-reactive, SiO.sub.2 layer.
Ultimately, an electrically conductive contact filling material such as tungsten would be provided for making electrical connection to the contact. However, tungsten adheres poorly to TiSi.sub.x. Additionally, it is desirable to prevent intermixing of the contact filling material with the silicide and underlying silicon. Accordingly, an intervening layer is typically provided to prevent the diffusion of the silicon and silicide with the plug filling metal, and to effectively adhere the plug filling metal to the underlying substrate. Such material is, accordingly, also electrically conductive and commonly referred to as a "barrier layer" due to the anti-diffusion properties.
One material of choice for use as a glue/diffusion barrier layer is titanium nitride. TiN is an attractive material as a contact diffusion barrier in silicon integrated circuits because it behaves as an impermeable barrier to silicon, and because the activation energy for the diffusion of other impurities is very high. TiN is also chemically thermodynamically very stable, and it exhibits typical low electrical resistivities of the transition metal carbides, borides, or nitrides.
TiN can be provided or formed on the substrate in one of the following manners: a) by evaporating Ti in an N.sub.2 ambient; b) reactively sputtering Ti in an Ar and N.sub.2 mixture; c) sputtering from a TiN target in an inert (Ar) ambient; d) sputter depositing Ti in an Ar ambient and converting it to TiN in a separate plasma nitridation step; or e) by low pressure chemical vapor deposition.
As device dimensions continue to shrink, adequate step coverage within the contact has become problematical with respect to certain deposition techniques. Chemical vapor deposition is known to deposit highly conformal layers, and would be preferable for this reason in depositing into deep, narrow contacts.
Organic compounds are commonly utilized as chemical vapor deposition precursors. One subclass of this group which is finding increasing use in chemical vapor deposition of metals and metal compounds are organometallic precursors. Specifically, an example is the reaction of a titanium organometaffic precursor of the formula Ti(N(CH.sub.3).sub.2).sub.4, named tetrakisdimethyl-amidotitanium (TDMAT), and ammonia or nitrogen in the presence of a carrier gas which reacts to produce TiN according to the following formula: EQU Ti(NR.sub.2).sub.4 +NH.sub.3 .fwdarw.TiN+organic by-products
Organometallic compounds contain a central or linking atom or ion (Ti in TDMAT) combined by coordinate bonds with a definite number of surrounding ligands, groups or molecules, at least one of which is organic (the (N(CH.sub.3).sub.2 groups in TDMAT). The central or linking atom as accepted within the art may not be a "metal" in the literal sense. As accepted within the art of organometallic compounds, the linking atom could be anything other than halogens, the noble gases, H, C, N, 0, P, S, Se, and Te.
The above and other chemical vapor deposition reactions involving organometallics are typically conducted at low pressures of less than 1 Torr. It is typically desirable in low pressure chemical vapor deposition processes to operate at as low a pressure as possible to assure complete evacuation of potentially undesirable reactive and contaminating components from the chamber. Even small amounts of these materials can result in a significant undesired increase in resistivity. For example, oxygen incorporation into the film before and after deposition results in higher resistivity. Additionally, it is believed that organic incorporation (specifically pure carbon or hydrocarbon incorporation) into the resultant film reduces density and resistivity. Such organic incorporation can result from carbon radicals from the organic portion of the precursor becoming incorporated into the film, as opposed to being expelled with the carrier gas. Carbon incorporation can also cause other undesired attributes in the deposited film, such as low density and poor long-term reliability.
Hydrogen is a known capable reactant with deposited carbon or metal carbides. Such will react with carbon atoms to form volatile hydrocarbons. Hydrogen atoms, radicals or ions are more reactive than molecular hydrogen in producing volatile hydrocarbons.
It would be desirable to improve upon these and other prior art chemical vapor deposition processes in methods of forming an electrical contact to a silicon substrate.